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In musical composition, the Opus number is the "work number" that is assigned to a composition, or to a set of compositions, to indicate the chronological order of the composer's production. Historically, although composers have inconsistently applied the opus number to their works, besides numeric cataloging, opus numbers are used to distinguish among compositions with similar titles.
To indicate the specific place of a given work within a music catalogue, the Opus number is paired with a cardinal number; for example, Beethoven's (Piano Sonata No. 14 in C-sharp minor, 1801) (nicknamed Moonlight Sonata) is "Opus 27, No. 2", which work-number identifies it as a companion piece to "Opus 27, No. 1" (Piano Sonata No. 13 in E-flat major, 1800–01), paired in same opus number, with both being subtitled Sonata quasi una Fantasia, the only two of the kind in all of Beethoven's 32 piano sonatas. Furthermore, the Piano Sonata, op. 27 N° 2, in C-sharp minor is also catalogued as "Sonata No. 14", because it is the fourteenth sonata composed by Ludwig van Beethoven.
Given composers' inconsistent assignment of opus-numbers, especially during the Baroque era (1600–1750) and the Classical era (1720–1830), musicologists developed other catalogue-number systems; among them the Bach-Werke-Verzeichnis (BWV-number), and the Köchel-Verzeichnis (K- and KV -numbers) with which are organised the works of Johann Sebastian Bach and Wolfgang Amadeus Mozart, respectively.
In the classical period, the Latin word opus ("work", "labour") was used to identify, list, and catalogue a work of art. By the fifteenth and sixteenth centuries, Italian and German composers respectively used the word opus to denote a musical composition and collections of music. In compositional practise, numbering musical works in chronological order dates from seventeenth-century Italy, especially Venice. In common usage, the word Opus is used to describe the best work of an artist with the term magnum opus.
Etymologically, the words opus (singular) and opera (plural) are related to the Latin words opera (singular) and operae (plural), the ancestor of the Italian words opera (singular) and opere (plural). In English usage, besides the word opus, the word opera occasionally was used to identify a musical work; yet, in contemporary usage, the word opera denotes the dramatic musical genre of opera, which was developed in Italy.
In the arts, an opus number usually denotes a work of musical composition, a practice and usage established in the seventeenth century when composers identified their works with an opus number. In the eighteenth century, publishers usually assigned opus numbers when publishing groups of like compositions, usually in sets of three, six or twelve compositions. Consequently, opus numbers are not usually in chronological order, unpublished compositions usually had no opus number, and numeration gaps and sequential duplications occurred when publishers issued contemporaneous editions of a composer’s works, as in the sets of string quartets by Joseph Haydn (1732–1809) and Ludwig van Beethoven (1770–1827); Haydn's Op. 76, the Erdödy quartets (1796–97), comprises six discrete quartets consecutively numbered Op. 76 No. 1 – Op. 76 No. 6; whilst Beethoven's Op. 59, the Rasumovsky quartets (1805–06), comprises String Quartet No. 7, String Quartet No. 8, and String Quartet No. 9.
19th century to date
From about 1800, composers, especially Beethoven, assigned an opus number to a work, and later to a set of works, especially songs and short piano pieces; however, composers’ inconsistent usages ended the correspondence between an opus number and the work’s publication date. Since approximately 1900, composers tended to assign an opus number to a composition, published or not. Early in his career, Beethoven selectively numbered his compositions (some published without opus numbers), yet in later years, he published early works with high opus numbers. Likewise, some posthumously published works were given high opus numbers by publishers, even though some of them were written early in Beethoven's career. Since his death in 1827, the un-numbered compositions have been catalogued and labelled with the German acronym WoO (Werk ohne Opuszahl), meaning "work without opus number". However, there are other catalogues of Beethoven's works – see Catalogues of Beethoven compositions.
The practice of enumerating a posthumous opus (“Op. posth.”) is noteworthy in the case of Felix Mendelssohn (1809–47); after his death, the heirs published many compositions with opus numbers that Mendelssohn did not assign them. In life, he published three symphonies (Symphony No. 1 in C minor, Op. 11; Symphony No. 2 in B flat major, Op. 52; and Symphony No. 3 in A minor, Op. 56); yet, he chronologically wrote symphonies between symphonies Nos. 1 and 2, which he withdrew for personal and compositional reasons; nevertheless, the Mendelssohn heirs published (and catalogued) them as the Italian Symphony No. 4 in A major, Op. 90, and as the Reformation Symphony No. 5 in D major and D minor, Op. 107.
While many of the works of Antonín Dvořák (1841–1904) were given opus numbers, these did not always bear a logical relationship to the order in which the works were written or published. To achieve better sales, some publishers, such as N. Simrock, preferred to present budding composers as being well established, by giving some relatively early works much higher opus numbers than their chronological order would merit. In other cases, Dvořák gave lower opus numbers to new works to be able to sell them to other publishers outside his contract obligations. This way it could happen that the same opus number was given to more than one of his works. Opus number 12, for example, was assigned, successively, to five different works (an opera, a concert overture, a string quartet, and two unrelated piano works). In other cases, the same work was given as many as three different opus numbers by different publishers. The sequential numbering of his symphonies has also been confused: (a) they were initially numbered by order of publication, not composition; (b) the first four symphonies to be composed were published after the last five; and (c) the last five symphonies were not published in order of composition. The New World Symphony originally was published as No. 5, later was known as No. 8, and definitively was renumbered as No. 9 in the critical editions published in the 1950s.
Other examples of composers' historically inconsistent opus-number usages include the cases of César Franck (1822–1890) and Béla Bartók (1881–1945), who initially numbered, but then stopped numbering their compositions. Carl Nielsen (1865–1931) and Paul Hindemith (1895–1963) were also inconsistent in their approaches. Sergei Prokofiev (1891–1953) was consistent and assigned an opus number to a composition before composing it; at his death, he left fragmentary and planned, but numbered, works. In revising a composition, Prokofiev occasionally assigned a new opus number to the revision; thus Symphony No. 4 is two thematically related but discrete works: Symphony No. 4, Op. 47, written in 1929; and Symphony No. 4, Op. 112, a large-scale revision written in 1947. Likewise, depending upon the edition, the original version of Piano Sonata No. 5 in C major, is catalogued both as Op. 38 and as Op. 135.
To manage inconsistent opus-number usages — especially by composers of the Baroque (1600–1750) and of the Classical (1720—1830) music eras — musicologists developed comprehensive and unambiguous catalogue number-systems for the works of composers such as:
- Johann Sebastian Bach — catalogued with a BWV-number; a Bach-Werke-Verzeichnis number assigned by Wolfgang Schmieder; however, older sources occasionally use S-numbers.
- Dietrich Buxtehude — catalogued with a BuxWV-number, a Buxtehude-Werke-Verzeichnis work number.
- Frédéric Chopin — three catalogue systems have been applied: (i) B-numbers, by Maurice J.E. Brown; (ii) KK-numbers, by Krystyna Kobylańska; and (iii) work-letters (A, C, D, E, P and S), by Józef Michał Chomiński. Generally, these alternative music-catalogue systems identified compositions that the composer had not numbered.
- Claude Debussy — identified with an L-number, per François Lesure's comprehensive catalogue.
- Antonín Dvořák — identified with a B-number, per Jarmil Burghauser's comprehensive catalogue; which resolved the problems of different and duplicate opus-numbers assigned by the publishers of Dvořák's music.
- Joseph Haydn — identified with a Hob.-number, per the 1957 catalogue by Anthony van Hoboken. Although he assigned Hoboken-numbers to the string quartets, those compositions usually are known by opus numbers.
- Franz Liszt — identified with an S-number, per the catalogue The Music of Liszt (1960), by Humphrey Searle.
- Wolfgang Amadeus Mozart — identified either with a K-number or with a KV-number (Köchel-Verzeichnis nummer), per the catalogue system of Ludwig Ritter von Köchel.
- Domenico Scarlatti — identified with three catalogue systems; (i) L-numbers, per the 1906 catalogue by Alessandro Longo; (ii) K-numbers and Kk-numbers, per the 1953 catalogue by Ralph Kirkpatrick; and (iii) P-numbers, per the 1967 catalogue by Giorgio Pestelli.